This invention relates to circularly polarized helix antennas and more particularly to an antenna system for producing a substantially broad beam width unidirectional radiation pattern or an omnidirectional radiation pattern in the horizontal plane using only three such helix antennas mounted to a triangular mast or tower.
In radio or television broadcasting, it is particularly desirable in many installations to broadcast an omnidirectional pattern so that all receivers at a given radial distance from the broadcaster's antenna receive essentially the same signal strength. Often the broadcasting antennas are mounted or are required to be mounted to a triangular tower which may serve as the support for other antennas. It is therefore desirable for the sake of cost to provide antennas which have a beam width and a structure such that when one is mounted to each side of a triangular tower essentially omnidirectional radiation takes place. When there is a requirement for the provision of circularly polarized radiation, additional problems are encountered in mounting the antennas to the reflective towers.
The use of a helical antenna to provide circularly polarized radiation is known in the state of the art. It is also known in the art to reduce backlobes by placing a helix forward of a ground reflector or within a conical or funnel-shaped reflector. It is known in the art that a helix within a funnel-shaped reflector produces a beam which is approximately the same beam width in all planes and in all polarizations. The phase center of a helix in a funnel-shaped reflector is located on the helix axis approximately in the plane of the aperture of the reflector. In order to produce a broad beam width pattern especially broad enough so that three about a tower produce an omnidirectional pattern, it is desirable to place the entire helix within the conical or funnel-shaped reflector and to shorten the helix and to shorten the reflector. It is known in the literature that a short helix on the order of a few wavelengths (two for example) without some form of end-loading cannot produce circularly polarized radiation. See "Some Aspects of the Design and Use of the Helical Antenna" by G. Svennerus, Proc. of International Congress, Ultra High Frequency Circuits, Paris, France, Oct. 21-26, 1957, Vol. 2. This is because the open end reflection causes a secondary circularly polarized wave to be radiated with a sense of rotation opposite to the primary radiated wave. However, there is no teaching in the prior art of an efficient end loading structure that permits a broad beam width pattern with a reasonably low axial ratio.